The present disclosure relates to carbon-based electrodes and, more specifically, to steam-activated carbon and related methods for preparing steam-activated carbon for use in making such electrodes. The disclosure also relates to high power density energy storage devices comprising carbon-based electrodes.
An ultracapacitor, also known as a double-layer capacitor, polarizes an electrolytic solution to store energy electrostatically. Though it is an electrochemical device, no chemical reactions are typically involved in the energy storage mechanism. The mechanism is reversible, which allows the ultracapacitor to be charged and discharged many times.
Ultracapacitors typically comprise two porous electrodes that are isolated from electrical contact with each other by a porous dielectric separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electric current from discharging the cell. Each electrode is typically in electrical contact with a current collector. The current collector, which can comprise a sheet or plate of electrically-conductive material (e.g., aluminum) can reduce ohmic losses while providing physical support for the porous electrode material.
Within an individual ultracapacitor cell, and under the influence of an applied electric potential, an ionic current flows due to the attraction of anions in the electrolyte to the positive electrode and cations to the negative electrode. Ionic charge can accumulate at each of the electrode surfaces to create charge layers at the solid-liquid interfaces. The accumulated charge is held at the respective interfaces by opposite charges in the solid electrode to generate an electrode potential. Generally, the potential increases as a linear function of the quantity of charged species (ions and radicals) stored at or on the electrodes.
During discharge of the cell, a potential across the electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode. Simultaneously, an electronic current can flow through an external circuit located between the current collectors. The external circuit can be used to power electrical devices.
The electrolyte serves as a promoter of ion conductivity, as a source of ions, and may serve as a binder for the carbon. The electrolyte typically comprises a salt dissolved in a suitable solvent. Suitable electrolyte salts include quaternary ammonium salts such as those disclosed in commonly-owned U.S. patent application Ser. No. 13/011,066, the disclosure of which is incorporated herein by reference in its entirety. An example quaternary ammonium salt is tetraethylammonium tetraflouroborate ((Et)4NBF4). An example solvent is acetonitrile.
Energy storage devices such as ultracapacitors may be used in a variety of applications, such as those where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. An important characteristic of an ultracapacitor is the energy density that it can provide. It has been demonstrated that the energy density of the device is largely determined by the properties of the carbon-based electrodes.
Carbon-based electrodes suitable for incorporation into high energy density devices are known. For example, high performance carbon materials, which form the basis of such electrodes, can be made via chemical activation of synthetic phenolic resin precursors. However, due to the relatively high costs of both the chemical activating agents and the synthetic resins, the cost of such activated carbon material and the resulting carbon-based electrodes can be high. Further, chemical activating agent may contribute unwanted impurities to the resulting activated carbon. Accordingly, it would be an advantage to provide a more economical process for preparing activated carbon material, which can be used to form carbon-based electrodes that enable higher energy density devices.